The present invention relates to an input signal level control device for a receiver. More particularly, the invention relates to an input signal level control device which is suitable for an FM receiver.
In a mobile FM receiver, the received electric field strength changes abruptly and greatly. When the field strength is large, the input high frequency amplifying element tends to be saturated so that the signal is subject to distortion by cross modulation and intermodulation.
In order to eliminate these difficulties, a technique has been employed in which an attenuator is interposed between the antenna terminal and the high frequency (radio frequency) amplifier circuit to attenuate the level of input signals to the high frequency amplifier circuit. The attenuator is activated by a manual switch to eliminate the above-described drawbacks which are caused when the field strength is high.
In accordance with another conventional technique, a so-called "AGC circuit" for automatically controlling the gain of the high frequency amplifier stage according to the received electric field strength is provided in the receiver. Howerver, that technique is still disadvantageous in that, when the electric field strength is high, the operating point of the amplifying elements is greatly shifted as a result of which the input signal is distorted. Furthermore, recently, a so-called "vari-cap" (variable capacitance) diode has frequently been employed in a tuning circuit. However, the tuning circuit suffers from the difficulty that, since it is employed as a tank circuit in the antenna input section, intermodulation which is caused by the vari-cap diode in the tank circuit cannot be eliminated.